Pfizer Shot Just 39% Effective Against Delta Infection, But Largely Prevents Severe Illness, Israel Study Suggests

Topline

Recent data from Israel’s health ministry suggests Pfizer’s Covid-19 vaccine is far less effective at preventing infection and symptomatic illness with the Delta variant than with previous strains of coronavirus, a finding that conflicts with other research indicating high levels of protection against the contagious variant as countries around the world struggle to contain new waves of infection.

The Pfizer Covid-19 vaccine appears to be significantly less effective against the Delta variant, … [+] data from Israel shows.

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Key Facts

A full course of the Pfizer-BioNTech vaccine was just 39% effective at preventing infections and 41% effective at preventing symptomatic infections caused by the Delta Covid-19 variant, according to Israel’s health ministry, down from early estimates of 64% two weeks ago.

The figures, based on data from an unspecified number of people between June 20 and July 17, are significantly lower than previous estimates of the vaccine’s efficacy against other variants, which initial clinical trials found to be 95%.

The Israel findings also conflict with several other studies assessing the vaccine’s performance against the Delta variant, which indicated only slightly diminished degrees of protection against infection and mild illness (between 80% and 90%), including peer reviewed research from Public Health England published Wednesday.

The vaccine still provides very high levels of protection against hospitalization (92%) and severe illness (91%) caused by the Delta variant, the ministry said.

In a statement, Pfizer and BioNTech noted that while real-world data from Israel show vaccine efficacy in preventing infection and symptomatic disease to decline six months post-vaccination, “efficacy in preventing serious illnesses remains high.”

Contra

It is possible Israel’s findings diverge from other studies in reflection of the fact they have had access to vaccines for longer than most other parts of the world. In other words, it is the most up to date and accurate assessment of the Pfizer shot against Delta. However, the Israel health ministry has not revealed the data behind their results and there are a number of issues in the way the study appears to have been conducted that could create uncertainty. The study did not, for example, take the steps needed to rule out alternative explanations for the lower efficacy, such as bias in who is tested for Covid-19.

Key Background

Israel was an early leader in the global effort to vaccinate against Covid-19, moving swiftly to secure supply by paying above market price and offering to share medical data with Pfizer. It was a success and the country dropped pandemic restrictions in May, though a resurgent wave of cases driven by Delta has put it on the brink of another lockdown. Israel is not the only country to struggle with new surges caused by Delta—which is more contagious than other variants circulating and possibly more dangerous—which has prompted new lockdowns around the world. The U.K., which has one of the highest case rates in the world, almost all of which are caused by Delta, is an exception. High vaccination rates are helping contribute towards a possible decoupling between infection, severe illness and death, prompting the government to drop almost all social restrictions on July 19.

What To Watch For

In mid July, Israel’s health ministry greenlit booster shots—likely a third Pfizer shot—for immunocompromised people over concerns they could still be vulnerable to the disease. This includes some cancer patients, organ transplant recipients and people who are HIV positive. The U.S. Centers for Disease Control and Prevention is considering a similar move amid an ongoing debate over whether booster shots should be made available to everyone.

Big Number

91%. That’s how effective the Pfizer vaccine is at preventing serious illness and hospitalization caused by the Delta variant after two doses, according to the Israeli study. This is only slightly lower than against other variants. As this is the primary purpose of a vaccine, it is still highly successful, though it does mean it will likely not be sufficient to stop outbreaks on its own.

Tangent

The Delta variant appears to be at least moderately resistant to many of the vaccines in widespread use, especially after just one shot. A recent study found the Johnson & Johnson shot to be just 33% effective at preventing symptomatic illness caused by the variant.